streams chapter6
TRANSCRIPT
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Stream Behavior Is Predictable
Streams seek a state of dynamic equilibrium
Equilibrium is a function of the flow and
sediment
Equilibrium is naturally associated with a main
channel and a flood-prone area
Effective (bankfull) discharge forms the
main channel Streams meander in a predictable manner
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Stream Stability
Natural stream channel stability is
achieved by allowing the river to develop a
stable dimension, pattern and profile such
that channel features are maintained andthe stream system neither aggrades nor
degrades (Leopold)
Correctly engineered rigid channels can be
part of a stable stream system.
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River Dimensions
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Velocity Distribution In A Channel
Depth-averaged velocity is above
the bed at about 0.4 times the depth
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Deep Overbank Flow
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Features Of A River
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Salt Creek (Hocking County Ohio)
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Flow in Streams
Effective Discharge
Shear Stresses
Sediment Transport
Bed Load Movement Land Use and Land Use Change
Open Channel Hydraulics
Resistance Equations
Compound Channel Pattern & Profile
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Effective Discharge The effective, dominant, and bankfull discharge are
often considered as synonymous.
The bankfull discharge is considered to be thechannel-forming or effective discharge (Leopold,
1994).
The bed load fraction of the total sediment load ismost influential in channel forming processes and
effective discharge (Emmett and Wolman, 2001).
The effective discharge typically occurs less than ahand full of times annually
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Bankfull Depth Versus Discharge
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Loramie Creek, Ohio
0
10000
20000
30000
40000
50000
0 500 1000 1500 2000 2500 3000
discharge rate
s
edimentdischarge(tonsx10
geomorphic
work
0
50
100
150
200
250
daysofoccurren
ce
A: sediment function (x 20) C: geomorphic work B: days
A:Sediment Transport Rate
B: Days Occurring C: Geomorphic Work
Effective Discharge
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Be d Load of Pre-Dev elopment and No C ontrol
of Post-dev elopme nt
0
1
2
3
4
5
6
0.1 1 10 100
Recurrence Interval (yr)
B
edLoad(m
3yr-1)
pre-development
no control
Effective Discharge
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Bed Material is Predictable
Bed MaterialBed Material
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Shear Stresses on the Bed and Bank
Mean bed material size is a function of the shear
stresses produced by the effective (critical) discharge
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Mean Particle vs. Tractive Force at Incipient Motion
0.1
1
10
100
1000
0.01 0.1 1 10 100
Tractive Force, T (kg/m2)
MeanDiameterBed
Material,(mm)
Upper Limit
Low Limit
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Andys 1 x 1 = 1 Rule
A 1 ft flow depth and a 1 % bed slope can
move 1 inch diameter bed material
A 4 ft flow depth and a 0.5 % bed slope can
move 2 inch diameter bed material(d50 approximately 2inch)
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Shear Stress on Banks
On straight banks they are 0.7 - 0.8
the mean bed shear stress
On bends they are 2-4 times the mean
bed shear when the ratio of the radius
of curvature to width is less than 5
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Bed Material Particle Sizes
Pebble Count, Small alluvial stream in the Midwest USA
0%
20%
40%
60%
80%
100%
0.01 0.10 1.00 10.00 100.00 1000.00
Particle Size (mm)
Percen
tFiner
Than
Cumulative Percent Percent Item
Based on a Wolman Pebble Count
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Sediment Transport
Wash Load
Suspended Load
Bed Load
Types
Methods Shear Stress
Power
Parametric
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Meanders
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Meander Length vs Channel Width
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One meander length is equal to
10-14 Bankfull Widths
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Meander Geometry
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Meandering adjusts the slope
for the best stability.
Slope = rise / run
Starts Here
Ends Here
Straighter, Steeper,
Faster
Meandering
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Flood Hydraulics within a Meander
Sellin and Willetts,Floodplain Processes, Walling, 1996
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Streamway Concept
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1989
1951
1966
1980
1997
Drainage Area
30 sq. mi.
Streamway
120 * DA 0.43 = 518 ft
Salt CreekVinton County, Ohio
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Channel Features: Profile (Slope)
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Pools and RifflesRiffle - Pools Features
(Spaced at 5-7 Bankfull Widths)
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Stream Changes Are Predictable
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Channel Evolution
Pre-Development
Downcutting
Widening
Restored Floodplain
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Bank Erosion Hilliard, Ohio
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Loss of Stream Equilibrium
Lost Floodplain AccessLost Floodplain Access
Channel
Resizing
Channel
Resizing
Loss of Buffering from SmallLoss of Buffering from Small
Streams and FloodplainsStreams and Floodplains
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Thank You!